In the measurement of fluid flow using acoustic or ultrasonic transit time differential measurement techniques it is known that the ultrasonic waves must have a wave length which is greater than the diameter of the conduit in which the fluid flow is to be measured. Under certain flow and acoustic conditions the phase shift and hence the change of arrival time of an acoustic plane wave transmitted along a conduit is proportional only to the flow rate and independent of the velocity profile, that is whether the fluid is flowing with laminar or turbulent flow. A flowmeter having certain characteristics capable of developing a plane wave of axisymmetric form has previously been disclosed in Patent No. GB2400439. In this document a plane wave is generated by annular piezoelectric acoustic transducers located at either end of a flow tube. This construction, which has the advantage of being a clean bore meter, provides a transit time difference measurement the value of which is proportional to the flow rate of fluid in the flow tube. This means that the flowmeter provides a linear performance throughout the laminar and turbulent flow regimes. In other words it is independent of Reynolds number.
In order to generate a planar wavefront, however, the wave length of the ultrasound must be greater than the diameter of the conduit as mentioned above. Thus, for a given flow tube diameter this sets the maximum frequency of ultrasound which can be used in the flowmeter for given material. For example, if the flow meter is used to determine the flow rate of water (in which the speed of sound is approximately 1500 m/s) in a flow meter the flow tube of which has a diameter of 6 mm, then the frequency of the ultrasound needed to satisfy the plane wave requirement is approximately 250 kHz. In larger flow tube sizes which are required for higher flow rates, the wavelength must be even lower and therefore the maximum frequency which can be used in the flow meter is also lower. This has significant disadvantages in ultrasonic flowmeters for use in situations where the flow rate is not high since the time difference between ultrasonic waves propagated upstream and downstream may be as small as 10 nanoseconds. Measuring such small time differences becomes increasingly difficult as the frequency decreases with the increase in wavelength.
The present invention seeks to overcome the limitations of the prior art ultrasonic flowmeters using the plane wave approach to the measurement of the flow rate whilst, at the same time, retaining the linearity of the flow meter response over the whole range of potential flow rates, and regardless of whether the fluid under test is flowing with turbulent or laminar flow.
As used in this specification, the term “acoustic” will be understood to include “ultrasonic” as applied to flowmeters.